Yeah, that 2-step thing is a drawback. Another drawback is if you accidentally go too dark w/it, you won't be able to lighten your hair again w/out taking drastic measures. So I was thinking about all this and have some other questions for you, before considering adding indigo:
You mentioned here or on another henna-related thread that you were using a "henna cream". Do you know what exactly is in that cream besides henna (if you're still using that form of henna)? Based on the info I got from Mehandi.com (whose founder has a PhD in henna, of all things lol), certain additives can significantly weaken the strength and color of henna. Another consideration is what type of henna you've been using, which reflects how high or low the lawsone content is. For ex., Jamila has less lawsone (therefore a weaker stain) than something like Rajasthani henna (very strong stain). And so Raj would probably offer better gray coverage. But with Raj you will get a burgundy/wine/auburn color - a deeper red than the Jamila. That's the trade off. So if you haven't already, you can try Raj w/out nothing added to it except warm water and some type of acid, for dye release ... if you're willing to go deep red/burgundy.